Gall wasp susceptibility of chestnut cultivars in an organic South Carolina orchard

Between years 1995 and 2000, 42 grafted Castanea spp. cultivars and 7 OP seedling populations were planted on a Cecil loamy sand soil near Seneca, South Carolina. The design was completely randomized with 2 replications per genotype and 4-20 trees for each seedling population. The orchard site was a former native forest and was not irrigated nor received fertilizer after establishment. Weeds were controlled the first few years, but no cover sprays were applied. Once established, the trees received no cultural practices other than mowing. C. sativa type cultivars eventually died from chestnut blight. In summer 2004, shoot galls of the Asian chestnut gall wasp (Dryocosmus kuriphilus) were found in the planting. Years 2006 and 2007 had the most infestation with galling noticeably decreasing in 2008. Subjective ratings of the level of canopy shoot galling from 0 (none) up to 5 (severe) were taken for each tree in March 2009. Galling was not noticeable again until 4-5 years later. The same ratings were used in February 2023 to score galling the previous 2 years. All trees were galled except ‘Lockwood’, which was the only C. crenata cultivar planted. The next least galled cultivars for both 2009 and 2023 were all C. mollissima (‘AU-Leader’, ‘AU-Cropper’, ‘Carr’, ‘Crane’, ‘Meiling’, and seedlings of ‘AU-Cropper’ and ‘Augenstein’). Hybrids ‘Colossal’ (C. sativa × C. crenata) and ‘Layeroka’ (C. mollissima × C. sativa) were severely galled and most died from blight. The gall wasp infestation began, peaked and collapsed within 6 years in the first occurrence and appeared to be ending within a similar time span in the second infestation. Though not verified, native or introduced parasitic wasps were suspected to have been biological control agents in the collapse of the infestations.